make the addons docker registry configurable

cluster/juju/layers/kubernetes-master/config.yaml

@@ -43,6 +43,10 @@ options:
       Audit webhook config passed to kube-apiserver via --audit-webhook-config-file.
       For more info, please refer to the upstream documentation at
       https://kubernetes.io/docs/tasks/debug-application-cluster/audit/
+  addons-registry:
+    type: string
+    default: ""
+    description: Specify the docker registry to use when applying addons
   enable-dashboard-addons:
     type: boolean
     default: True
@@ -74,7 +78,7 @@ options:
       privileged mode by default. If "false", kube-apiserver will never run in
       privileged mode. If "auto", kube-apiserver will not run in privileged
       mode by default, but will switch to privileged mode if gpu hardware is
-      detected on a worker node. 
+      detected on a worker node.
   enable-nvidia-plugin:
     type: string
     default: "auto"

cluster/juju/layers/kubernetes-master/reactive/kubernetes_master.py

@@ -719,6 +719,7 @@ def configure_cdk_addons():
     gpuEnable = (get_version('kube-apiserver') >= (1, 9) and
                  load_gpu_plugin == "auto" and
                  is_state('kubernetes-master.gpu.enabled'))
+    registry = hookenv.config('addons-registry')
     dbEnabled = str(hookenv.config('enable-dashboard-addons')).lower()
     dnsEnabled = str(hookenv.config('enable-kube-dns')).lower()
     metricsEnabled = str(hookenv.config('enable-metrics')).lower()
@@ -726,6 +727,7 @@ def configure_cdk_addons():
         'arch=' + arch(),
         'dns-ip=' + get_deprecated_dns_ip(),
         'dns-domain=' + hookenv.config('dns_domain'),
+        'registry=' + registry,
         'enable-dashboard=' + dbEnabled,
         'enable-kube-dns=' + dnsEnabled,
         'enable-metrics=' + metricsEnabled,